The invention relates to a method for producing a reference image for pattern recognition tasks.
Pattern recognition tasks are becoming increasingly important. They play a role in conjunction with industrial robots for highly automated production tasks, and in particular for automatic identification of personnel. The fields of use in that case include, inter alia, access monitoring, automatic teller machines with cash dispensers, and all types of communications equipment.
Suitable personal features, i.e., biometric features for identification include, for example, the pattern of the blood vessels in the retina, the structure of the iris, the length and shape of the fingers, the face, the voice, or else fingerprints. The options for identifying persons on the basis of these features are described, for example, in IEEE Spectrum, February 1994, xe2x80x9cIt had to be youxe2x80x9d. Fingerprints are thereby probably the best researched, and thus most reliable, features. They can also be determined in a manner which is convenient for the user while, for example, the pattern of the retina can be recorded only by using a procedure which is unpleasant for the person being identified, and which can thus be used only in those cases in which this effect is irrelevant, or is even desirable.
The uniqueness and individuality of fingerprints has been verified by careful series of experiments and statistics; it is thus certain that each person""s fingerprints are individually specific. Even in cases of identical twins, the fingerprints are not identical, although they appear to be very similar. The pattern of the fingerprints is formed by the so-called papillary lines or ridges which are produced by the interlocking of two layers of skin.
In this case, the so-called minutiae are primarily of interest for the identification process, these being end points, branches, islands or other singularities. The fingerprint to be identified is thus investigated to determine which minutiae it has, the minutiae which have been determined are allocated to a type, and their position and orientation with respect to one another are established.
The minutiae obtained in this way describe the fingerprint completely. They are used as the basis for comparing the fingerprint to be identified with stored reference patterns.
The matching probability of the recognition process depends entirely on the quality of these reference patterns.
A secondary aspect of quality is in this case the size of the reference image.
In order to produce a reference image which is as large as possible, it has become known from U.S. Pat. No. 4,933,976 for a sequence of overlapping individual images to be recorded from a finger while it is being rolled over a conventional prismatic assembly and for these individual images then to be joined together for a larger overall image. In that process, it is assumed that the individual images have only been linearly displaced with respect to one another, and not rotated.
According to the disclosure in the international publication WO 98/02844, the compilation of a mosaic-like image from individual images is known for general image processing applications.
The object of the invention is to provide a method of producing a reference image for pattern recognition which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this kind, and which allows high quality reference patterns to be produced to form images from which symbolic line formation can be obtained in a stable and reproducible manner.
With the above and other objects in view there is provided, in accordance with the invention, a method of producing a reference image for pattern recognition tasks, which comprising the following method steps:
recording a plurality of at least partially overlapping images of a pattern to be recognized;
producing a symbolic intermediate representation for each of the images by digitizing and establishing probable lines or shapes;
comparing the symbolic intermediate representations of two images with one another, and determining a relative shift and rotation of the images with respect to one another;
transforming the two intermediate representations coincide in an overlapping area and determining a quality of the match;
if the quality of the match is adequate, synthesizing the two intermediate representations to form an overall image;
comparing the overall image with a respective further intermediate representation and producing from this a further overall image, until an overall image is obtained which comprises all the intermediate representations with a useable matching level, and which is used as a reference image.
In other words, the objects of the invention are satisfied with the novel method for producing a reference image for pattern recognition tasks, comprising the following method steps:
a number of at least partially overlapping images of the pattern which is to be recognized is recorded,
a symbolic intermediate representation is produced for each of the recorded images,
the symbolic intermediate representations of two images are compared with one another, and the relative shift and rotation of the images with respect to one another are determined,
transformation is used to make the two intermediate representations coincide in the overlapping area and to determine the quality of the match,
if the quality of the match is adequate, the two intermediate representations are synthesized to form an overall image,
the overall image is compared with in each case one further intermediate representation and a further overall image is produced from this until, in the end, an overall image is obtained which comprises all the intermediate representations with a useable matching level, and which is used as a reference image.
The probability of the area of the image in question being contained completely in the xe2x80x9cvirtualxe2x80x9d area of the reference can thus be made as high as desired.
The quality of the features which are located in regions of the reference image which have been recorded more than once is considerably increased by averaging.
The invention can be used particularly advantageously if the pattern to be recognized is a fingerprint.
The process for adding to and improving the reference image can also be continued in the course of the recognition processes by adding each recognized fingerprintxe2x80x94each recognized patternxe2x88x92to the reference image, thus increasing the quality of the reference. Furthermore, in this way, limited changes such as minor injuries in the area of the fingerprint, are recorded and taken into account.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for producing a reference image for pattern recognition tasks, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.